This invention relates to a method of preparing an improved coating composition to a wet surface or object. The coating comprises a two-part system of an epoxy resin and any of a special group of amine curing agents. In one embodiment, in preparing a coating for application to a damp surface, the resin and an amine-epoxy resin adduct curing agent are separately made into water emulsions, the emulsions are then mixed together, and the admixture thereafter coated onto the substrate.
Two-part epoxy resin systems are well known as coatings for their outstanding adhesion to the substrates and for their toughness and chemical resistance. There are literally hundreds of such systems available which typically consist of part A, an epoxy resin, and part B, an amine-based compound. However, these prior-art systems are unable to obtain satisfactory results when coating a wet surface, for they do not adhere properly unless the surface is completely dry.
Robert M. Jorda described in U.S. Pat. No. 3,160,518 a process for applying a coating to surfaces immersed in water. In his process he used a coating comprising a polyepoxide and a considerable stoichiometric excess of a polyamine, the excess being at least 5% but more typically 50%. The excess was required because without it, the epoxy could not be completely cured and so could be washed away. It has now been determined that much, and in some cases even most, of the polyamine of Jorda dissolves in the water before it can react with the epoxy. How much will so dissolve depends on whether the water is agitated (e.g., if it is in a flowing stream or in the ocean near the surf), how soluble the particular amine is, the temperature of the water, the rapidity of the reaction between the amine and the epoxy and therefore how reactive the particular epoxy is, and so on. The finally reacted epoxy may be weakened or affected by retaining an excess of polyamine or may be weakened or affected by being deficient in polyamine, so that some of the epoxy resin is unreacted. The results are thus unpredictable, uncertain, unreliable. One can hardly know in advance all the affecting conditions well enough to be sure that he will have exactly the right degree of excess of polyamines to achieve whatever results are optimum for the particular epoxy and particular polyamine. Thus, Jorda did not solve the problem completely, however much improvement he may have achieved over what had been done before. And the Jorda process always wastes material, by using more amine that what is really needed for the reaction.
Some epoxy-amine materials (including some of the Jorda materials) are, moreover, water sensitive. Even when they have been fully reacted, when put in water or left in water, these materials may blister or change color, become opaque, tend to flake, or become soft.